In usage(), don't use `usagestr' as the `fprintf' format string.

Instead, print the usage string using "%s".  This is a safety bug found by
-Wformat.

os/imagezip/imagezip.c

@@ -928,7 +928,7 @@ char *usagestr =
 void
 usage()
 {
-	fprintf(stderr, usagestr);
+	fprintf(stderr, "%s", usagestr);
 	exit(1);
 }